Short- and medium-range structural order around cations in glasses: a multidisciplinary approach

The structural environment of cations at a short- and medium-range scale ma y be investigated either by spectroscopic methods or by radiation diffracti on giving either a description of the geometry and symmetry of the cationic site, including the nature of the chemical bond, or a chemically resolved radial distribution function. Cations exhibit several original structural p roperties in oxide glasses. Short-range order is characterized by unusual c oordination numbers, such as five-coordinated sites or tetrahedral sites wh ich are in a network forming position, with the relative proportion of thes e sites depending on glass composition. Oxide glasses can also exhibit elem ents with unusual oxidation states, such as pentavalent uranium. The determ ination of the sites occupied by the elements in their different oxidation states allows to rationalize the chemical dependence of redox equilibria, w hich is the way to predict Fe behavior in magmatic silicate systems. Severa l experimental data lend support that cations are located in domains extend ing up to more than 8 Angstrom radius, in which cationic polyhedra may be l inked together either by edges or by corners. In low alkali berate glasses, transition elements such as Co, Ni, Zn exist in peculiar highly ordered do mains corresponding to the presence of rigid berate units. Strong differenc es are observed between modifying and charge compensating cations, either c oncerning site geometry or medium-range organization. The use of numerical models for experimental data inversion allows to rationalize the structural behavior of the various glass components. (C) 2001 Academie des sciences/E ditions scientifiques et medicales Elsevier SAS.